Nitric acid plant and control system



Dec- 21, 1954 J. M. RIBBLE ErAL NITRIC ACID PLINI.` AND CONTROL SYSTEMFiled May 22 1950 INVENTORS J. M. RIBBLE u. J. Fox By E. EDMuNDs JR.

A T TOR/VE V5 nited States Patent O NITRIC ACID PLANT AND CONTROL SYSTEMJack M. Ribble and Jack J. Fox, Bartlesville, kla and Edward Edmunds,Jr., Albuquerque, N. Mex., assxgnors to Phillips Petroleum Company, acorporation of Delaware Application May 22, 1950, Serial No. 163,444

7 Claims. (Cl. 23--260) This invention relates to the manufacture ofnitric acid. In a specific aspect this invention relates to .a methodand apparatus for the production of nitric acid from ammonia.

It is well known that, in the production of nitric acid from ammonia,the ammonia gas is rst catalytically oxidized to oxides of nitrogen,principally nitric oxide, and the oxides of nitrogen produced togetherwith an excess of air, with which the nitric oxide reacts to formnitrogen dioxide, are subsequently absorbed in water to produce nitricacid. In the usual process for the production of nitric acid fromammonia, the gases, rich in nitric oxide, from the oxidation chambers inwhich the ammonia is oxidized, are passed under atmospheric or higherpressure together with an excess of air into an oxidation chamber, wherethe nitric oxide is oxidized to nitrogen dioxide. The nitrogendioxide-containing gas mixture is then passed through an absorptioncolumn in counterilow with a current of water, and the resulting nitricacid is recovered from the process.

It is recognized that to obtain maximum eiiiciency from the process itis essential that the ratio of ammonia to air in the ammonia oxidationreaction and the amount of air consumed in the entire process be rigidlycontrolled. Unintentional changes in llow rates, caused by` such simplefactors as a drop in the voltage or frequency of the electrical powersupply, may elect a variation in the ratio of ammonia to air with itsconsequent effect upon the efiiciency of the process. Also, intentionalchanges in ilow rates to control the rate of manufacture are sometimesnecessary. Heretofore, when such a change in rate of manufacture was tobe made, it was necessary to regulate the amount of ammonia, to changethe amount of air to that necessary under the new conditions, and toguard against overheating of the catalyst, and to manually andexperimentally harmonize all these factors was diiicult andtime-consuming.

An object of this invention is to generally improve the method andapparatus for the manufacture of nitric acid by ammonia oxidation.

` A further object of this invention is to overcome the foregoingdifliculties and to provide automatic regulation of the process.

A further object of this invention is to provide a method forautomatically controlling the amount of oxygen consumed in such aprocess.

A further object of this invention is to provide a method forautomatically controlling the amount of air introduced to the ammoniaoxidation reactor.

A further object of this invention is to provide a method forautomatically controlling the amount of air introduced to the nitricoxide oxidation zone to form nitrogen dioxide.

Further and additional objectsof our invention will be readily apparentfrom the disclosure and description of the drawing hereinbelow.

To accomplish these objects and other objects that will hereinafterappear we employ our invention consisting of the method and theapparatus elements and their relationship to each other. Our inventionwill be described in detail with reference to the accompanying drawlngwhich is a ow diagram of a preferred manner of carrying out ourinvention. Such conventional equipment as pumps, lters, and the like,other than those required for an understanding of our' invention, havenot been included in this drawing, but the inclusion of such equipmentis within the scope of our invention.

ice

Referring now to the accompanying drawing, ammonia y,enters the systemvia line 1 and passes to vaporizer 2, provided with steam coils or othersuitable means for heating and vaporizing the ammonia. Ammonia gasleaves vaporizer 2 via line 3 for mixing with air prior to entrance intothe oxidation reaction. Air enters the system via line 4, and at least aportion thereof passes into power recovery compressor 5 wherein theexpansion of the residual gases or off-gases from the system is employedto compress the air supplied to the system. Compressed air fromcompressor 5 passes via line 6 to cooler 7 and thence to air receiver 8.Any air, entering the system, that is not passed through compressor 5,is passed to receiver 8 directly via line 9. Air from receiver 8 iswithdrawn and passed via line 10, and a portion thereof is passed vialine 11 to the ammonia oxidation reaction. In actual operation the airin line 10 is at a pressure of about pounds per square inch gauge and atemperature of 49 C., and the ammonia in line 3 is at a pressure ofabout pounds per square inch gauge and a temperature of 71 C.

The ammonia and air from the oxidation reaction passing via lines 3 and11 respectively are mixed prior to entrance into the oxidation reactionas described hereinbelow. To effect the oxidation reaction the ratio ofammonia to air in the reaction mixture is closely controlled in order toobtain maximum conversion eciency and to minimize the danger of theformation of explosive mixtures. Air-ammonia mixtures containing lessthan 8 or more than 10.5 volume per cent ammonia show too low aconversion eiciency, and the preferred concentration of ammonia in theair-ammonia mixture is within the range of 9.5 to 10.3 volume per cent.In order t0 control the air-ammonia mixture within these limits a ratiocontrol device is employed to proportion the volume of the ammonia gaspassing via line 3 to the volume of process air passing via line 11. Anysuitable means for controlling the volumetric ratio of air to ammoniamay be used. For example, in some instances a single differential ratloilow controller is satisfactory. We prefer to employ a recorder receiverliow controller, shown on the drawing as controller 12, to accomplishthe desired control. Controller 12 is set to maintain a concentration ofammonia. of 9.5 to 10.3 volume per cent in the airammoma mixture, andthis setting is automatically ad- )usted within this range in accordancewith the oxygen content of the oli-gas from the system in a manner to bedescribed hereinbelow. Lines 3 and 11 are provided with ilowtransmitters 13 and 14 respectively, and these transrmtters areconnected directly to controller 12. Accordingly, the ow rates in lines3 and 11 -are transmitted to c ontroller 12 which operates motor valve15 in ammonia lme 3 either electrically, mechanically or pneumatcally.For each change in the ow rate in air line 11, as transmitted tocontroller 12 by transmitter 14, controller 12 automatically adjusts theammonia ow rate in line 3 to maintain a constant air-ammonia mix. Thusthe desired air to ammonia ratio is maintained constant at all owvolumes within the range of the measuring instruments. It is desirableto employ a ow controller that operates over a wide range in order thatratio control is obtained with the low ilow rates at start-up as well asthe high ow rates at maximum production. Line 11 is provided with valve16 which is employed to manually control the air ow rate in that line.Ammonia line 3 is provided with by-pass line 17 and valve 18 which isordinarily employed when the ammonia flow is on manual operation. Suchoperation is employed when the process is being started or when therequired ammonia volume exceeds the capacity of motor valve 15.

Prior to being mixed with ammonia, the air for the oxidation reaction isconducted via line 11 to air heater 19 for preheating of the air. Inheater 19 the air is passed into heat exchange relationship with theefuent from the oxidation reaction. A portion of the air in line 11passes to mixer 22 via line 20 without passing through heater 19, andconsequently, this portion of air is not preheated. The remaining airfor the oxidation reaction enters heater 19, and its temperature iselevated therein. The heated air is withdrawn from heater 19 via line21, and this air, as well as the air in line 20, is passed to mixer 22containing damper 22A. The adjustment of damper 22A is controlledl by'the catalysttemperature in the oxidation reactor in order to obtain thedesired preheat temperature of the air` for the oxidation reaction.After mixing: 'of thel hot and. coolerfair in mixer 22 the air passesvialine123l` to mixer 24'where it is=admixedvy with the; ammonia to beoxidized; In: view of the operation of controller 12, previouslydescribed",I air and ammonia are. admixed4 in mixer 24in the propervolumetric proportions for the oxidation' reaction. Admixed air andammoniapass via line 25 to` oxidation reactor 26.

Reactor 26 is provided with asuitable'catalyst for eifectingy theoxidation reaction.` The preferred type of catalyst is aplatinumcatalyst. PlatinumI aloneor platinum in an alloy' with a noblemetal,` suchv as copper, nickel, cobalt, silver, tungsten, vanadium,andxthe.like,.may be'ernployed.` The preferredltype-fof. catalyst is aplatinum-rhodium` alloy containing' about 10 per cent rhodium. Thislatter catalyst is usually used in the: form of an 80-mesh gauze withthe gauzein either single or multiple'layers. Actually, variousphyshical forms of the catalyst may be used instead of the wire gauzeform. For example, the catalyst may be in the form of perforated.sheets, narrow strips of metallic turnings, impregnated masses, and thelike. The wire gauze type is preferred, and it may be in any suitableform, such as cylindrical baskets,.tlat sheets, and the like.

In reactor 26 ammoniais oxidized with air in accordance with thefollowing equation The reaction is exothermic and it may start at atemperature as low as 500 C. Temperature increases favor the formationof nitric oxide, but at temperature above 970 C. the formationofnitrogen as a product of the reaction is increased. The nitrogen, formsas a result of the oxidationfof ammonia to water and nitrogen instead ofnitric oxide. Reaction temperatures within the range of 900 to 970 C.are employed, and the minimum allowable conversion' of ammonia is.96 percent. Conversion rates increase with reactions temperature up to 950 to960 C. at which temperature conversion rate increases become negligible.It is preferred not to operate at temperatures above 930.I C., exceptfor short periods of time, due to the catalyst losses` at highertemperatures. The most preferred temperature is within the range of 900to 910 C. Since high4 pressures tend to increaese the formation ofnitrogen at the expense of the desired nitric oxides, pressures inexcess of 125 pounds per square inch are usually not employed;

The temperature inreactor 26 is maintainediwithin thedesired-temperature by controlling the preheat' temperature of the airvprior touits admixture withy ammonia. A temperatureresponsivedevice,.such asa Chromel-Alumel thermo-couple, is placed' in the wiregauzeI catalyst or slightly below'it. The device is connected torecording temperature controller 27 which by electrical, mechanical orother suitable means' adjusts damper 22A in mixer 22. Whenever the gauzetemperature in reactor 26'fluctuates from a predetermined temperaturewithin the range al'- readyy discussed, controller 27 adjusts damper 22Ain a manner thatthe preheat temperature of the air flowing in line 23 isincreased or decreased, as the case'may be. If the temperature inreactor 26 becomes too high, damper 22A is adjusted to admit more coolair via line 20 and less hot air via line 21. On the other hand, ifthetemp erature in reactor 26'becomes too low, damperv 22A is adjustedto admitmore hot. air via line 21 and less cool air via line 20, andthus` the preheat temperaturevof the air-ammonia mixture is increasedlwith a resulting increase in the reactor temperature. TheY temperatureof the preheated air is usually adjusted to maintain the preheattemperature of the ,air ammonia mixture within the range of 280 to 310'C., althoughhigh and lower preheat temperatures may beused.

The gaseous efuent leaving the gauze catalyst in reactor 26 contains byvolume, about 10 per cent nitric oxide and about 7 per cent oxygen inaddition to steam andnitrogen, and the gas-is ata temperature of aboutA900 C. The

lower portion of reactor 26' is surrounded by a cooling.

jacket, that cools the gaseous reaction etiluent to about 750 C. Thereaction eluent is withdrawn from reactor 26 via line 28 and thenceviaheater 19 where itY serves to: preheat the air for the oxidationreaction. In air heaterr 19 the reaction efuent is cooled to about 500C. From heater 19` thegaseous eluentis withdrawny viay lineL 29,- andthen it is further cooledTto about 300 C. by passage through heatexchanger 30 in heat exchange relationship with a part or all of theoff-gas from the system. Efuent gases are withdrawn from. heat exchanger30 via line 31 to condenser 32 where the eilent is further cooled to atemperature no-higher than 150. C. Condenser 32 is a series of coolingtubes over which water is owed to absorb heat and effect the necessarycooling. Uponv being cooled to a temperature no higher than 150 C., thenitric oxide in the reaction eluent isI oxidized` by the oxygen alsopresent in that effluent in accordance with the following equation:

This reaction is exothermic, and it is necessary for the cooling waterowing over the condensing tubes to remove this heat as well as to coolthe reaction euent Water to effect the heat removal iny condenser 32enters the system via line 33 and thence it flows downwardly over thecondenser tubes. About per centof the nitric oxide entering condenser 32is oxidized to nitrogenf dioxide which reacts with water inthe oxidationefuent to form nitric acid in accordance` with the following equation:

This reaction is alsoexothermic and it forms additional quantities ofnitric oxide tobe oxidized with oxygen to nitrogen dioxide.

At an intermediate point in condenser 32 the oxidation euent passingtherethrough is passedvia line 34' to acid'y separator 35 where weaknitric acid is separated and gaseousfoxidation effluent is returned tocondenser 32 viay line 36. Weak nitric acid. is withdrawn from separator35 via line 37, and it is introducedl to absorption column 38J on thatplate or tray which contains nitric acidfofI the same strength as thatinline 37. Introduction of the weak acid ata lower point in column 38'causesA av dilution of the nitric acid product, and introduction of theweak acid at a higher point in the column places anexcessive burdeny onthe top of the column and it leads to high yieldl losses in the stack oroff-gases.

Eluent from condenser 32 is withdrawn via line' 39 and then passed toabsorption column 38'. In actual operation the' feed to column 38 is ata temperature of about 30 C. Column 38 is a bubbler cap absorptioncolumn containing. a series of plates or trays, and water or steamcondensate is introduced to the top of the column via line 40.r Thewater passes down through. the column where it reacts withk nitrogen.dioxide to formI nitric acid andy where: it absorbs the nitric acid`thus formed. The reactionl of nitrogenV dioxide and water forms nitricoxide, and this nitric oxide as well as the nitric oxide entering thecolumn via line 39 is oxidized to nitrogen dioxide for further reactionwith water. Column 38 is operated at thel lowest temperatureobtainablewith ordinary coolingwater, usual'- ly within the range of 20to 40 C., andthe pressure in column 38 is about 80 pounds per squareinch'.

The: nitric acid withdrawnA from` column: 38` contains some nitricoxide, and consequently the acid has* a slight brownish.L color. Inorder to remove this color the acid is passed to bleachingcolumn- 41 towhich air is introduced for oxidation of thenitric oxide to nitrogendioxide. Column 41 isa ring packed column connected with column 38.Nitric acid trickles over the packing against the up,- ward flowing airstream which passes up` into' column 38 for oxidation of nitric oxidetherein. column 41" is employed to oxidize-nitric oxide' in column 41and in column 38;

The bleaching action in column 41 is effected at a temperature abovethat at which` column 38 is operatedl andv usually within the range of40to 50 C. To obtain this temperature the air flowingin line 10andenteringco'lumn.

temperature in: column 41'., This method of Aoperating-i has adisadvantage initliatl it changes the.ratiojofhammoniato air in the feedto reactor- 26fafter. the feed4 mixture has- Thus, air entering beenautomatically adjusted to a predetermined ratio by controller 12.Controller 12 can be set to compensate for the withdrawal of air vialine 42, but each time a change occurs in the rate of withdrawal vialine 42 it is necessary to adjust controller 12 accordingly. When theair entering column 41 is preheated, as described, nitric acid passesfrom column 38 to column 41 via line 43. However, we prefer to operateour process by introducing cold air to column 41 and without removingair from heater 19 via line 42. ln our preferred method of operation,nitric acid is withdrawn from column 38 via line 44, and it is thenpassed to steam heater 45 where it is heated to a temperature of 40 to50 C. prior to passage into column 41 via line 46. A temperatureresponsive device in line 46 is connected to temperature controller 47which operates motor valve 48 in steam line 49. lnthis manner thequantity of steam entering heater 45 is automatically controlled in sucha manner that the temperature of the heated nitric acid is within thedesired limits. Steam condensate is removed from heater 45 via line 50.1t is desirable to maintain the temperature of the nitric acid passingto column 41 below 50 C. in order that the corrosive eiect of the acidis kept at a minimum. Bleached nitric acid is withdrawn from column 41via line 51 as a product of the process.

ln order that the nitric acid produced in our process can have thedesired concentration, the quantity of water or steam condensateentering the system via line 40 is automatically controlled. To elfectthis control, recording density controller 52 is connected to line 46and valve 52 in line 40. Whenever a change occurs in the density of thenitric acid in line 46, indicating a change in concentration, controller52 adjusts the setting of valve 53 to increase or decrease, as the casemay be, the quantity of Water entering column 38 via liney 40 in orderthat the concentration ofthe nitric acid may be decreased or increased,as desired. In this manner any acid concentration can be obtained.Usually the concentration is at least 50 per cent, preferably at least60 per cent, and more preferably 62 to 66 per cent.

Gaseous eiuent from column 38, containing nitrogen, oxygen and nitrogenoxides, is withdrawn via line 54. For e'icient operation of our processit is essential that the oxygen content of the eiuent from column 38 bemaintained within close limits, preferably not above 5 volume per cent,and more preferably within the range of 2 to 3 volume per cent. When theoxygen concentration drops below 2 volume per cent, excessive amounts ofnitrogen oxides are lost in the gaseous eluent from column 38, and, whenthe oxygen concentration rises above 5 volume per cent, the power costsfor operation of the system become excessive. Thus, the oxygen contentof the gaseous el'iluent must be rigidly controlled.

We have found that the oxygen content of the efluent gas from column 38can be employed to control automatically the oxygen consumption in thesystem. As a consequence of the control of the oxygen consumption in thesystem, the oxygen content of the efuent gas is maintained within thedesired limits for eicient and economical operation. Control of theoxygen consumption in the system can be effected by two methods, i. e.,the ratio of ammonia to air in the reaction mixture entering reactor 26can be automatically controlled or the quantity of air entering column41 can be automatically controlled. To operate an ammonia oxidationprocess in accordance with our invention we employ oxygen recordercontroller 55 in line 54. This controller produces a continuous recordof the oxygen content of the effluent gas from column 38, and it isconnected to either controller 12 or valve 56 in line 10, either ofwhich is activated by controller 55, Whenever the oxygen content of theeiiluent gas is outside the desired limits. The activation of controller12 or valve 56 is by either electrical, mechanical or pneumatic means.

In one embodiment of our invention the amount of air entering column 41via line 10 and valve 56 is automatically controlled in accordance withthe oxygen content of the gas in line 54. Controller 12 is set to obtainthe desired air-ammonia mixture, and reactor26 is operated at thereaction conditions set forth above. Accordingly, whenever the oxygencontent of the efiluent gas in line 54 exceeds or falls below thedesired limits, this oxygen content is an indication that either toomuch or too little oxygen is entering column 41 for oxidation of nitricoxide in columns 38 and 41. Therefore,lcontrol1er 55 automaticallyadjusts valve 56 to increase or decrease, as the case may be, the rateof ilow of air to column 41 until the oxygen content of the eliluent gasreturns to the desired concentration. he setting of valve 56 remains asso adjusted until a further adjustment is necessitated by a variation inthe oxygen content recorded by controller 55.

In another aspect of our invention, oxygen recorder controller isemployed to adjust the setting of controller 12. In this aspect the tlowof air to column 41 via line 10 1s manually adjusted to permit the entryof suicient oxygen to oxidize nitric oxide in column 41 and to obtain asubstantially water-white or straw-color product therefrom. Then, whenthe oxygen content of the eflluent gas in line 54 exceeds the desiredupper limit, controller 55 adjusts the setting of controller 12 toincrease the ammonia concentration of the air-ammonia mixture for theoxidation reaction, and consequently the concentration of excess oxygenin line 54 is decreased. On the other hand, when the oxygen content ofthe eiiluent decreases to less than the desired minimum limit,controller 55 adjusts the setting of controller 12 to decrease theammonia concentration in the feed to reactor 26, and consequently theconcentration of excess of oxygen in line 54 is increased. Since theammonia oxidation reaction is exothermic, any increase or decrease ofthe ammonia concentration in the feed to the reaction will have aneffect upon the reaction temperature in reactor 26. In our process, anysuch ternperature eiect is compensated for automatically by con' troller27 adjusting the preheat temperature of the inlet air for the reaction.v

A portion or all of the eiiluent gas passing via line 54 is passed vialine 57 to heat exchanger 30, and it is then returned to line 54 vialine 58. Thus, the efuent gas serves to cool the reaction gases from theoxidationreaction prior to passage of the former via line 54 to powerrecovery unit 5, where it serves to compress the inlet air to thesystem. In actual operation the use of power recovery unit 5 on theexhaust gas from the system decreases the overall power consumption forthe processv by as much as 50 per cent. Exhaust gases from the systemare vented via line 59.

Since the gaseous eluent from column 38 contains some nitric acid, line54 is provided with separator or mist collector 64 for separation ofnitric acid via line 65 in order to prevent corrosion of equipment,particularly power recovery unit 5, by this nitric acid.

`In the operation of the disclosed process it is important thatvariations in pressure be kept at a minimum, primarily to reducevariations in the operating conditions of reactor 26 and also to assurea constant pressure to the power recovery unit for its operation. It isessential that the pressures of the air and ammonia entering the oxidation reactor be maintained at a constant value, since variations in oneor the other of these feed pressures affect the respective volumesentering the reactor and seriously affect the proper operation ofcontroller 12. In order to insure a minimum pressure variation in thesystem, line 54 is provided with recording pressure controller 60 whichautomatically controls the setting of-valve 61 and thus maintains aconstant back pressure on the system within the range of to 120 poundsper square inch. After passage through valve 61, eluent gases are ventedfrom the system via line 63. In actual operation, there is a pressuredrop through the system, and this pressure drop acts as a reducing valveto smooth out any variations in pressure. The minimum pressure drop thatshould be maintained is l5 pounds per square inch in order to minimizeeiects upon the temperature in reactor 26 caused by variations inpressure.

Numerous variations and modifications of our invention will be readilyapparent to those skilled in the art from our disclosure hereinabove.

We claim:

l. In a plant for producing nitric acid including a catalytic converter,a line supplying ammonia to said converter, a cool air supply line, aheated air supply line joining said cool air line, an air line leadingfrom the juncture of said two air lines to said converter, an air heaterin said heated air supply line, a condenser, an absorption column, aneluent line extending from said converter to said condenser to supply aneffluent comprising nitrogen oxides, water, and unreacted oxygen to saidcondenser, means for transferring condensate and uncondensed vapors fromsaidcondenser to said absorption column wherein' nitrogen oxides areconverted to nitric acid, a lgaseous eluent 'conduit extending from thetop of said column, and means for withdrawing nitricfacid from thebottom of said column, 'a control system which comprises, incombination, a damper at the junction 'of said cool air line 'and saidheated air .line movable to regulate the pro'portion lof heated air toco'ol air fed 'to said converter, a temperature controller having asensing element 4in 'said converter and operatively connected to saiddamper so fas to maintain a constant predetermined converter temperature-by regulation of said damper, 'an oxygen recorder vcontroller having asensing element in said gaseous `eflluent conduit, a ratio 4controllerhaving sensing elements in said ammonia line and said cool air line, andoperatively connected to a control valve in one of said lines so as tocontrol the ammonia-air ratio in said converter, said ratio controllerbeing operatively connected to said oxygen recorder controller so as tomaintain a predetermined efuent oxygen content through control of said'ammonia-air rat-io.

2. In a Vplant for producing nitric acid including a catalyticconverter, a line supplying ammonia to said converter, a line supplyingair to said converter, a'condenser, an absorption column, an effluent'line extending from said converter to said condenser to supply an euentcomprising nitric acid, water and unreacted oxygen to said condenser,means for transferring condensate and uncondensed vapors from saidcondenser to said absorption column wherein nitrogen oxides wareconverted to nitric acid, a gaseous eflluent conduit extending from thetop of said column, a bleaching column, means for withdrawing nitricacid from the bottom of said absorptioncolumn and passing it to saidbleaching column, and a line supplying air to said bleaching column, acontrol system which comprises, in combination, aratio controller havingsensing elements in said ammonia line and said converter air line, andoperatively connected to a control 'valve in one of said lines so as tocontrol the ammonia-air ratio in said converter, said ratio controllerhaving va 'settable element capable of a variation to change lsaid'ammomia-air ratio, a valve in said air supply line to the bleachingcolumn having an element settable to `vary fthe proportion of air fed tothe bleaching column, an oxygen recorder controller having a sensingelement in said 4gaseous eluent conduit, said oxygen recorder`controller being operatively connected to one of said settable elementsso as to maintain a predetermined `efliuent oxygen contentthroughcontrol of the oxygen fed to the system.

3. In a plant for producing nitric acid including a catalytic converter,a line supplying ammonia to said con'- verter, a line supplying air to.said converter, a condenser, an absorption column, an effluent lineextending from said converter to said condenser to supply aneffluent-comprising nitrogen oxides, water and unreacted oxygen to saidcondenser, means for transferring condensate and uncondensed vapors fromsaid condenser to said absorption column wherein nitrogen oxides areconverted to nitric acid, a gaseous Veiluent conduit extending from thetop of said column, and means for withdrawing nitric acid from thebottom of said column, a control system which comprises, in combination,a ratio controller having sensing elements in said ammonia line and saidair line, and operatively connected to a control valve -in one of saidlines so as to control the ammonia-air ratio in said converter, anoxygen recorder controller having a sensing element in said gaseousefuent conduit, said ratio controller being operatively 'connected tosaid oxygen Vrecorder controller so as to maintain a predeterminedeffluent oxygen content through control of said ammonia-air ratio.

4. In a plant for producing nitric acid Iincluding 'a catalyticconverter, means for supplying a 'mixture of air and ammonia to saidconverter, a condenser, an absorption column, a bleaching column, anefuent line extending from said converter t'o 'said condenser Vto supplyan `eiiluent comprising nitrogen oxides, water, and unreacted oxygen tosaid condenser, 'means for transferring cooled effluent from 'saidcondenser to said absorption column wherein nirtogen oxides areconverted to `nitric acid, a gaseous efuent conduit extending from thetop of said absorption column, means for withdrawing'nitric acid fromthe bottom of said absorption column and passing it to said bleachingcolumn, means .for withdrawing bleached nitric acid from said bleachingcolumn, and

a-line forfsupplying 'air to Asaid -blea'chinglcolumm a control systemwhich comprises, in combination, a valve in' said-air lin'e to 'regulatethe amount of air passing to said bleaching column, and an Ioxygenvrecorder controller having 'a sensing element in said gaseous effluentconduit, 'said recorder 'controller being loperatively 'connected tosaid valve 'so as to maintain a predetermined effluent oxygen contentthrough control of the oxygen fed to said bleaching column.

l5. lIn a plant for producing nitric acid including a catalyticconverter, va line supplying ammonia to said converter, a line supplyingair to said converter, a condenser, an absorption column, a bleachingcolumn, an etlluent line extending from said converter to said condenserto supply an effluent comprising nitrogen oxides, water, and

f' unreacted oxygen-to said condenser, means for transferring cooled'eiuent 'from said condenser to said abso'rption 'column wherein*nitrogen oxides are converted to nitric acid, a gaseous etiluentJconduit extending from the top of 'said absorption column, means lforwithdrawing nitric acid from the bottom -of said absorption column andtransferring it to "said bleaching column, means for withdrawingbleached 'nitric acid from said bleaching column, and a line forsupplying fair to said bleaching column, a control system whichcomprises, in combination, a valve in one of said air lines, and anoxygen recorder controller vhaving y'a sensing element in said gaseouseffluent conduit, 4,said recorder controller being operatively connectedto said valve soas fto ymaintain a predetermined eiliuent oxygen contentthrough control of the oxygen introduced linto the system through saidvalve.

46. In 'an absorption 'system for producing nitric acid, in combination,an .absorption column, a bleaching column, ,means for introducingnitrogen oxides, oxygen and water to :said absorption column to producenitric acid therein, .1a heater, a line for circulating a heating mediumy line through regulation of the amount of heating medium circulatedthrough :said Iheater, means for admitting air to said bleaching column,and means for withdrawing bleached .nitric acid therefrom.

A7. yIn an absorption system for producing nitric acid, in combination,.an absorption column, a bleaching column, means for introducingnitrogen oxides and water to said absorption column to .produce -nitricacid therein,l

heater to said vbleaching column to transfer heated nitric acid to thebleaching column, a tempertaure controller having a sensing element insaid last-mentioned line and operatively connected to said valve to.maintain a constant ypredetermined temperature in said last-mentionedline through regulation of 'the amount of heating medium circulatedthrough said heater, means for admitting air to said ,bleaching column,Vmeans for withdrawing bleached nitric acid therefrom, a line supplyingwater in a-uid state to thetop of said absorption column, a secondcontrol 'valve in said last-mentioned line, a density controller having'a sensing element in the line connecting said heater with-saidbleaching column, said density controller being operatively connected tosaid second control valve to maintain a constant density of the heatednitric acid 'in the lin'e connecting "said heater with said bleachingcolumn by regulation `ot the water fed to said absorption column.

yReferences ACited n'th'e tile of this patent UNITED STATES PATENTSNumber Name Date 1,594,264 Howard July 27, 1926 `1,644,123 Griswold c.Oct. 4, 1927 "1,709,042 ISiebert t. Apr. 16, 1929 ,1,770,059 :BarberJuly 9, 1930 1,923,865 fHandforth Aug. v22, y1933I (Otherlreferences`-on followingpage) UNITED STATES PATENTS OTHER REFERENCES Number NameDate Taylor et al.: Manufacture of Nitric Acid by the 2,046,162Handforth et al. June 30, 1936 Oxidation of Ammonia, Ind. and Eng.Chem., vol. 23, 2,088,057 Handforth July 27, 1937 5 No. 8, pagesS60-865, August 1931. 2,090,921 Titlestad Aug, 24, 1937 Chem. and Met.Reports, Chem. and Met. Engineering, 2,135,733 Richardson Nov. 8, 1938May 1943, pages 97-114. 2,185,579 Beekhuis Jan. 2, 1940 ChemicalEngineering, vol. 55, No. 11, pages 106, 2,226,113 Chastain Dec. 24,1940 107, November 1948. 2,393,362 Gerhold Jan. 22, 1946 10 Webb:Absorption of Nitrous Gases, pages 330-336. 2,417,877 Lewis Mar. 25,1947 Longmans, Green and Co., N. Y. C., 1923.

2,462,995 Ritzmann Mar. 1, 1949

3. IN A PLANT FOR PRODUCING NITRIC ACID INCLUDING A CATALYTIC CONVERTER,A LINE SUPPLYING AMMONIA TO SAID CONVERTER, A LINE SUPPLYING AIR TO SAIDCONVERTER, A CONDENSER, AND ABSORPTION COLUMN, AN EFFLUENT LINEEXTENDING FROM SAID CONVERTER TO SAID CONDENSER TO SUPPLY AN EFFLUENTCOMPRISING NITROGEN OXIDES, WATER AND UNREACTED OXYGEN TO SAIDCONDENSER, MEANS FOR TRANSFERRING CONDENSATE AND UNCONDENSED VAPORS FROMSAID CONDENSER TO SAID ABSORPTION COLUMN WHEREIN NITROGEN OXIDES ARECONVERTED TO NITRIC ACID, A GASEOUS EFFLUENT CONDUIT EXTENDING FROM THETOP OF SAID COLUMN, AND MEANS FOR WITHDRAWING NITRIC ACID FROM THEBOTTOM OF SAID COLUMN, A CONTROL SYSTEM WHICH COMPRISES, IN COMBINATIONA RATIO CONTROLLER HAVING SENSING ELEMENTS IN SAID AMMONIA LINE AND SAIDAIR LINE AND OPERATIVELY CONNECTED TO A CONTROL VALVE IN ONE OF SAIDLINES SO AS TO CONTROL THE AMMONIA-AIR RATIO IN SAID CONVERTER, ANOXYGEN RECORDER CONTROLLER HAVING A SENSING ELEMENT IN SAID GASEOUSEFFLUENT CONDUIT, SAID RATIO CONTROLLER BEING OPERATIVELY CONNECTED TOSAID OXYGEN RECORDER CONTROLLER SO AS TO MAINTAIN A PREDETERMINEDEFFLUENT OXYGEN CONTENT THROUGH CONTROL OF SAID AMMONIA-AIR RATIO.